You're not completely clear:
Are you talking about minimizing the browser window or minimizing an Ext.Window in the desktop example?
(I'll assume the first one)

Firefox sometimes looses the cursor of an input field in absolutely positioned container after an application switch (this is a really old bug that is still not fixed).
To get the cursor back you have to do another application switch (Alt-Tab, Alt-Tab).

I'm talking about an Ext.Window with a formpanel with and editor in it. I don't use the Desktop.

I tried to put hideMode:'offsets' on the window but the problem still persists. The iframe is still there when I look with firebug but there is no html markup inside anymore. If I switch to source mode the source appears.